1. Field of the Invention
The present invention concerns a semiconductor optical device and, more particularly, it relates to a semiconductor optical device suitable for application to semiconductor laser devices and semiconductor light modulation devices used for optical fiber communication, optical information processing and optical measurement.
2. Description of the Related Art
A semiconductor optical device having a quantum well represented by MQW (multiple quantum well) lasers; has been noted as a fundamental technique in the optical communication in the next generation, in which effects such as an increase in the relaxation oscillation frequency, and a reduction in the spectral linewidth reflecting the quantum size effect are expected.
Values of the band-edge discontinuity energy in the quantum well region and the barrier region in conventional InGaAsP series QW (quantum well) lasers are greater on the side of the valence band than on the side of the conduction band. That is, in the QW structure using InGaAs(P) for the quantum well region and InP or InGaAsP for the barrier region, the well depth .DELTA.Ec on the side of the conduction band is smaller than the well depth .DELTA.Ev on the side of the valence band (that is .DELTA.Ec&lt;.DELTA.Ev). Accordingly, the well depth sensed by electrons (effective well depth to the electrons) is shallower than the well depth sensed by holes (effective well depth to the holes). The ratio, i.e., the ratio of the value for the band-edge discontinuity energy, particularly, the value for the conduction band discontinuity: EQU .DELTA.Ec/(.DELTA.Ec+.DELTA.Ev)
is about from 0.3 to 0.4 in this system. As an example, it is described as 0.38 in Applied Physics Letters, Vol. 51 (1987), pages 24-26.
As in the prior art described above, in the Qw structure having a relationship: .DELTA.Ev&gt;.DELTA.Ec, there is a problem that it is difficult to inject holes to each of the well regions.
This will be explained referring to band diagrams shown in FIG. 3A and 3B. Both of FIGS. 3A and 3B use InGaAs as the quantum well region, and InP is used in FIG. 3A and InGaAsP is used in FIG. 3B for the barrier region. Referring at first to FIG. 3A, the difference of the band gap between the quantum well region and the barrier region is large (about 610 meV) and .DELTA.Ec is about 232 meV and .DELTA.Ev is 378 meV. In this case, although electrons and holes are sufficiently quantitized, since .DELTA.Ev is too great, the holes can be injected into the first well region but can not easily be injected into the second and the third well region, because the well depth on the side .DELTA.Ev of the valence band is too great and the holes can not override it. This leads to a remarkable increase of a threshold current. In view of the above, the barrier region has been made of AnGaAsP for reducing .DELTA.Ev (refer to FIG. 3B). Thus, the holes can be injected to each of the wells as shown in the figure, whereas the well depth .DELTA.Ec on the side of the conduction band is also reduced. Then, the distribution of electrons with small effective mass exceeds the energy of the well region and electrons are not confined in the well region and are present as shown by the hatched line in FIG. 3B. Thus, the degree of electron quantitization is reduced and the quantum size effect of the quantum well can not fully be developed. In this case, although the threshold current is decreased, the quantum size effect is reduced to bring about a technical problem that the purpose of introducing the quantum well structure is defeated.
Further, in a semiconductor laser device using a laminated semiconductor structure having a quantum well, it has been known to provide a so-called strained super lattice structure utilizing the lattice constant mismatching of a semiconductor layer constituting the laminated semiconductor structure, for example, as disclosed in IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol, QE-24 (1988), pages 1605-1613. In this prior art, if the degree of lattice mismatching, that is, the amount of strain is increased, dislocations occur making it impossible to form a semiconductor layer (quantum well region) having a thickness greater than a predetermined value which is referred to as a critical thickness. For instance, as shown by solid circles in FIG. 11, if the strain, that is, deviation of the lattice constant in the quantum well region relative to the lattice constant in the substrate is present in an amount of 2%, it is difficult to form a quantum well region having a thickness of greater than 100 .ANG. and, if the strain is present in an amount of 4%, it is difficult to form a quantum well region having a thickness of greater than 40 .ANG.. Generally, in the strained super lattice structure, optical properties are improved as the strain is increased as compared with a no-strain state. However, in the prior art, if the strain is increased, it correspondingly requires a reduction of the thickness of the quantum well region as described above. Accordingly, it is not possible to optionally and independently set the thickness and the strain for the quantum well region. Description will be made referring to a typical example. Now, considering intrusion of electrons to the barrier region, the range of the film thickness of the quantum well region for utilizing the quantum size effect most effectively is within a range about from 50 to 150 .ANG., but it is not possible in this case to introduce more than about 2% of strain (refer to FIG. 11). This remarkably hinders the degree of freedom for the design of the quantum well structure.